The present invention relates to partially enhanced drill bits for drilling in ground formations.
In percussive drilling in subterranean rock formations, a drill bit is alternatively rotated and impacted, whereby pieces of rock are broken away. The impacts can be generated locally by a down-the-hole piston, or from the ground surface.
A conventional drill bit for percussive drilling comprises a steel body with button inserts mounted on the drilling face. The buttons are arranged in generally concentric annular rows, with at least the outer row known as the gauge row, and the other rows known as the face rows. On larger bits, several outer rows may be referred to as gauge rows. The bit includes flushing holes for conducting flushing fluid that conducts-away the cuttings.
In order to minimize the rate of wear of the buttons, the buttons are usually formed of a hard material such as cemented carbide. The gauge row buttons perform the most work, and thus diamond-enhanced buttons are occasionally used as gauge buttons, i.e., buttons having a diamond layer applied thereto, such as natural or synthetic diamond. Bits having diamond enhanced gauge buttons, and non-diamond enhanced face buttons are called "partially-enhanced" bits. Bits wherein both the face buttons and gauge buttons are diamond-enhanced are called "fully enhanced" bits.
Conventional partially-enhanced bits 1, 2 and 3 are shown in FIGS. 1A-B, 2A-B and 3A-B, respectively, the bits being provided with diamond-enhanced gauge buttons 4D and cemented carbide (non-enhanced) buttons 4C.
FIGS. 4A and 4B depict how the buttons of a partially enhanced bit 5 become worn, the buttons including diamond-enhanced outer or gauge buttons 6D and non-enhanced (carbide) face buttons 6C. A fresh, unworn bit is depicted in FIG. 4A. The bit 5 is depicted in FIG. 4B after the non-enhanced buttons have been worn (the broken lines indicating the original shape of the buttons). Note that the enhanced buttons 6D are essentially unworn, due to their diamond layers. The wearing of the non-enhanced face buttons requires that the bit be pulled from the ground so that the non-enhanced face buttons can be re-shaped. As shown in FIG. 4B, it is necessary to remove some of the steel bit body to expose more of the face button to enable the face button to be re-shaped. Re-shaping is performed a number of times until the face buttons are so worn that the gauge buttons become overloaded and break.
The above-described problem could be addressed by using diamond-enhanced buttons in the face, but this solution greatly increases the cost of the bit.
It would be desirable to alleviate the problem in a more cost-effective way.